The basic principle of fiberoptic transducers revolves around shining a light on a reflective surface and measuring how much light is reflected back to a fiberoptic rod. The amount of light that is reflected is determined by the distance that the reflective surface is located from the light source and light collector. In general, fiberoptic transducers use one or more fibers to channel light from the light source to a reflective surface and use one or more fiberoptic rods to collect the reflected light from the reflective surface and channel it to a measuring device.
In the prior art, one type of pressure measuring device, as disclosed in U.S. Pat. No. 4,787,396, uses a diaphragm that is reflective or has reflective elements attached to it. As the pressure moves the diaphragm, the reflecting surface is moved towards or away from the light transmitting rods or optical fibers to change the amount of reflected light in proportion to the pressure applied to the diaphragm. Generally, the sensitivity of a pressure transducer is governed by the flexibility of the diaphragm, by its thickness by and its surface area. For medical applications, it is desirable to make the transducers as small as possible. As the diameter of the diaphragm decreases, the sensitivity of the diaphragm to pressure decreases in a way that is inversely proportional to the square of the diameter. This can be partially offset by decreasing the thickness of the diaphragm. There are limits, however. The first limitation is a physical limitation in making progressively thinner diaphragms without making them porous. The other limitation is the cost of economically forming very thin diaphragms.
To attempt to overcome these disadvantages, transducers such as those disclosed in U.S. Pat. No. 5,065,010 were developed. In this patent, the diaphragm is a miniature bellows which can be altered in its length, number of convolutions and the material thickness to vary its sensitivity. The major disadvantage of this particular pressure transducer is that a cavity is created in which fluid is collected. In the case where the fluid is blood, it tends to coagulate in this cavity. This eventually leads to error in reading the correct average pressure as well as a deterioration of the frequency response of the transducer. This damping or degradation of the measured pressure wave form may cause misdiagnosis of a patient's true medical condition.
Another limitation of prior art pressure transducers involves the end use of the transducer. If a transducer having a diaphragm is inserted in a blood vessel or other fluid conduit, there is an inherent error in measuring the correct pressure due to errors introduced by the flow of fluid impinging normal to the diaphragm. The static pressure is increased by the dynamic pressure of the flowing fluid against the diaphragm.
A further improvement was disclosed in U.S. Pat. No. 4,991,590 in which a cylindrically shaped pressure sensor has a side window and an L-shaped thin leaf having two sections including a reflector section arranged at an angle of approximately 90.degree. to the other section, the other section being movably situated within the side window with one end of the reflector section being attached to the outer cylinder wall opposite to the window. A membrane surrounds the cylinder to seal the pressure sensing mechanism from fluid and to permit the transmission of pressure to the L-shaped leaf. One of the disadvantages with this transducer is that the leg of the L-shaped leaf containing the reflector portion is cemented or otherwise affixed in a slit in the housing wall opposite the side window. This requires a bending moment about the base of the leg that contains the reflective element. Since the L-shaped leaf is made of a flexible material, the bending moment of the leg containing the reflective element may cause the reflective element itself to bend, thus causing a nonlinearity in the reading taken. Further, the sensitivity of the instrument is not easily adjusted since the thin leaf has a long horizontal leg to provide enough bending moment to flex the vertical leg that has the reflective element and that is attached at the end thereof to the side wall.
The present invention overcomes the disadvantages of the prior art by providing a pressure transducer that consists of a tubular member in which an opening or cavity is provided in the side of the tubular member. An L-shaped beam has one leg attached horizontally to the tubular member in a cantilever fashion such that the other leg extends vertically into the cavity. A reflective surface is formed on at least one side of the vertically extending leg and a flexible membrane covers the cavity and is in contact with the horizontal leg of the L-shaped beam. At least one fiberoptic rod is mounted in the housing in spaced relationship with the vertical leg for transmitting light to and receiving light from the reflective surface such that when pressure is applied to the membrane, the horizontal leg is flexed inwardly thus changing the angular position of the vertical leg with respect to the fiberoptic rod, thereby changing the amount of reflected light in an amount proportional to the applied pressure. A recessed ledge is formed around the opening or cavity in the housing for receiving the horizontal leg of the L-shaped leaf and the flexible diaphragm that covers the cavity or opening. With a slot in the horizontal leg of the L-shaped leaf and an orifice in the recessed ledge around the cavity or opening for receiving an attachment device through the slot in the horizontal leg of the leaf, more or less of the horizontal leg may be caused to extend over the edge of the recessed ledge a distance L1. By adjusting the length L1 of the horizontal leg in relation to the length of the vertical leg, L2, the sensitivity of the probe or transducer may be varied or altered. This occurs because there is a geometric relationship between the distances L1 and L2 which affects the displacement of the reflecting surface in relation to the displacement of the end of the horizontal portion of the cantilevered beam caused by a pressure exerting a force on the horizontal beam. Thus, the displacement of the outer end of the horizontal leg can be mechanically amplified or deamplified by choosing different ratios of L1:L2.
Further, with this design, the problem of fluid flow perpendicular to the diaphragm is avoided since the membrane is on the side of the tubular member and is essentially flat. Also, no stagnant area is present for fluid to gather or, in the case of blood, to coagulate. It also enables the transducer to be more easily configured to suit sensitivity requirements.
With the present invention, the L-shaped leaf can be mounted to the recessed ledge to establish a first pressure sensitivity over a first range of flexure of the horizontal leg and a second pressure sensitivity over a second range of flexure of the horizontal leg. This is accomplished by forming an angled portion on the outer end of the horizontal leg and attaching the outer end of the horizontal leg to the recessed ledge such that the remaining portion of the horizontal leg, in its normal position, is angled upwardly and does not touch the edge of the recessed ledge, whereby a first pressure deflects the angled portion of the horizontal leg until it contacts the edge of the recessed ledge. A second greater pressure further deflects the portion of the horizontal leg beyond and below the edge of the ledge, thus creating first and second pressure sensitivities of the transducer. The cantilevered beam is attached at a point on the recessed ledge of the tubular member such that the length L1 of the horizontal leg is the effective length of the horizontal leg and is slightly angled apart from the attaching ledge or attaching surface. As increasing pressure is applied, the horizontal leg deflects, causing the angle to decrease until the horizontal leg touches the edge of the recessed ledge. At this point, the effective length of the beam has been shortened to a length L3. With an effective length of L1, the transducer is more sensitive than when the effective length is L3. It is therefore possible to construct a transducer which is more accurate when measuring low pressures, such as intracranial and left atrial pressures, and less accurate when measuring higher pressures, such as arterial cardiovascular pressures. As an example, a one millimeter of mercury error when measuring 10 millimeters of intracranial pressure, a 10% error, is more significant than a 3 millimeter error in reading a cardiovascular pressure of 100 millimeters, a 3% error.
Thus, it is an object of the present invention to provide a fiberoptic pressure transducer that avoids error caused by fluid flow impinging normal to the diaphragm or membrane.
It is also an object of the present invention to provide a fiberoptic transducer that does not have an open cavity in which fluid may be collected and, in the case of blood, coagulated which leads to error in reading the correct average pressure as well as causing deterioration of the frequency response of the transducer.
It is yet another object of the present invention to provide a pressure transducer in which the sensitivity of the transducer is tailored to the application.
It is still another object of the present invention to provide a pressure transducer in which two or more levels of sensitivities for two or more ranges of pressure may be provided simultaneously.
It is yet another object of the present invention to provide a pressure transducer in which an L-shaped beam has one leg attached horizontally to a tubular member in cantilevered fashion such that the other leg extends vertically into a cavity in the tubular member, the vertical leg having a reflective surface thereon. A flexible membrane covers the cavity and is in contact with the horizontal leg of the L-shaped beam. At least one fiberoptic rod is mounted in the housing in spaced relationship with the vertical leg for transmitting light to and receiving light from the reflective surface such that pressure applied to the membrane flexes the horizontal leg inwardly to change the angular position of the vertical leg with respect to the fiberoptic rod and change the amount of received reflected light proportional to the applied pressure.